1. Field of the Invention
The present invention relates to an apparatus and method for transmitting and receiving signals using multi-antennas.
2. Description of the Related Art
Methods for transmitting and receiving signals using multi-antennas have a higher channel-use efficiency than conventional methods using a single antenna. Such methods using multi-antennas include a method that obtains transmission diversity by using multi-antennas on a transmitting side, and a method that obtains receipt diversity by using multi-antennas on a receiving side. In addition, there have been provided a method that obtains transmission and receipt diversities by using multi-antennas on both a transmitting side and a receiving side, and a method that forms parallel channels to transmit and receive signals after optimizing a transmitting side through beamforming.
The method that uses multi-antennas on both the transmitting side and receiving side performs beamforming using these multi-antennas, and then performs water filling, and optimization of the transmitting side provides a higher transmission speed or a higher capacity under the same channel environments. In a system using frequency division duplex FDD, however, in which separate channels (bandwidths) are used for transmitting and receiving, the receiving channel information differs from the transmitting channel information. Therefore, the receiving side should estimate channel information and send the estimated information to the transmitting side, so that the transmitting side can “learn” the transmitting channel information. Consequently, the number of antennas increases, and twice as much channel information as the number of antennas is required. If the channel environment changes continuously, the transmission period of channel information to the transmitting side becomes faster, resulting in deterioration of channel-use-efficiency.
To solve these problems, in a general cellular communication system including one base station and a plurality of user communication terminals within one cell, the base station presumes a certain transmitting channel. The base station then performs beamforming to optimize itself to the presumed transmitting channel. Each communication terminal measures the effective signal-to-noise-ratio (ESNR) of a received signal, and sends the measured ESNR to the base station. Then, the base station compares the ESNR values received from the respective communication terminals, and transmits signals to a communication terminal having the best channel state, during a predetermined time period. Then, during a next predetermined time period, the base station again searches for a communication terminal having the best channel state, and transmits signals to the communication terminal having the best state. This is called a multi-user transmission method. However, it is difficult to optimize the base station in a multi-user transmission method because the base station performs beamforming and transmits the beams.